1. Field of the Invention
The present invention relates to a focus adjustment device for adjusting a focus of a focus lens in an image acquisition apparatus and a focus adjustment method for doing the same.
2. Description of the Related Art
Image acquisition apparatuses have an automatic focus adjustment function (what is called an autofocus (AF) function) to adjust a focus of a focus lens on a subject automatically. The AF function includes various types of systems. As one of the systems, a phase difference AF system is known. In this system, focusing is performed by the pupil division type phase difference detection system in response to a signal output from a focus detecting pixel row. In the phase difference AF system, overlap control may be performed. In the overlap control, a lens driving amount is updated by detecting a phase difference (calculating a defocus amount) during the driving of the focus lens as well as the stopping of the focus lens. The larger the defocus amount, the lower the accuracy of the lens driving amount to be calculated. Thus, the overlap control makes it possible to obtain a more correct focusing state by correcting the lens driving amount in sequence as the defocus amount is decreased by driving the focus lens.
It is desirable to drive the focus lens at high speed to shorten AF time when the foregoing overlap control is performed. If, however, the focus lens is driven at high speed, the overlap control cannot be performed in time and thus the focus lens may pass the focusing point. Jpn. Pat. Appln. KOKAI Publication No. 2011-232684 to Karasawa proposes a method of selecting a low driving speed of a focus lens when the focus lens is driven to move close to the focusing point.